Bleaching of mechanical pulps with hydrosulfite in the presence of an alkali metal silikate

ABSTRACT

A PROCESS IS DESCRIBED IN WHICH MECHANICAL PULPS ARE BLEACHED WITH SODIUM OR ZINC HYDROSULFITE IN THE PRESENCE OF SODIUM SILICATE. THE ADDITION OF SILICATE INCREASES THE BRIGHTNESS GAIN OBTAINED BY HYDROSULFITE ALONE AND REDUCES THE GRINDING POWDER CONSUMPTION. SILICATE CAN THUS REPLACE SODIUM TRIPOLYPHOSPHATE FOR THIS USE AND THEREBY ELIMINATE PHOSPHATE POLLUTION DUE TO NEWSPRINT OR OTHER PAPER MILL EFFLUENTS.

FIPBSU'? 3,709,779 Patented Jan. 9, 1973 ABSTRACT OF THE DISCLOSURE Aprocess is described in which mechanical pulps are bleached with sodiumor zinc hydrosulfite in the presence of sodium silicate. The addition ofsilicate increases the brightness gain obtained by hydrosulfite aloneand reduces the grinding power consumption. Silicate can thus replacesodium tripolyphosphate for this use and thereby eliminate phosphatepollution due to newsprint or other paper mill efliuents.

The present invention relates to the bleaching of mechanical pulps, andmore particularly to a method of bleaching such pulps without usingsodium tripolyphosphate.

Wood which contains varying amounts of cellulose, hemicelluloses, ligninand small amounts of other materials can be defiberized into amechanical pulp by two processes. In the first process the debarked logsof wood, softwood or hardwood, are ground against pulp stones to convertthem into a fibrous form known in the industry as stone groundwood. Inthe second process debarked wood is converted into chips which are inturn converted into the fibrous form by rubbing between two metalplates; this type of mechanical pulp is commonly known as refinergroundwood in the pulp and paper industry.

In the preparation of the above types of mechanical pulp no majorcomponent of wood is lost, although a small amount of water solublematerial may be dissolved. In general the yield of the mechanical pulpsobtained by the above two processes varies between 95 and 100%. A smallvariation in the refining process is also practiced in the industry: thewood chips for preparation of refiner groundwood are first soaked in asolution of sodium sulfite or a weak aqueous solution of sodiumhydroxide or mixtures of the two. This softens the chips to some extentand disc refining is facilitated. The pulp thus obtained is sometimescalled chemi-groundwood and is obtained in slightly poorer yield thanregular refiner groundwood. All the above types of pulp shown identicalbleachability behaviour with hydrosulfites and peroxides.

A mechanical pulp obtained by any of the above methods is commonly usedin combination with various chemical pulps for the manufacture ofnewsprint, catalog paper, book paper, etc For many of these uses thereflectivity, or brightness, of the mechanical pulps prepared from manyspecies of wood is satisfactory, while pulps from other woods are toodark-to be used without further bleaching or brightening. For economicreasons it is necessary to bleach these pulps without removingsubstantial amounts of wood components, i.e., by use of the so-calledlignin-preserving bleaching agents. The most common bleaching agents forthis purpose are sodium or zinc hydrosulfite, also referred to as sodiumor zinc dithionite, 'and the peroxides of hydrogen or sodium. Both thesereagents are used commercially. Bleaching can borohydride and thioureadioxide, but they are not commercially used for var-ions reasons.

Of the two common reagents used commercially, hydrosulfite and peroxide,the former is used in more mills than the latter. Hydrosulfites can beadded to the mechanical pulp at any point between the grinders orrefiners and the paper machine. Usually it is added at the deckers wherethe consistency and temperature is quite suitable for hydrosulfitebleaching. In addition, the pulp stays in the deckers long enough forhydrosulfite to act. Hence no special equipment is required. On theother hand, peroxide bleaching can be performed only in specialbleaching towers where the consistency of pulp (about 25% for optimumresults) is much higher than anywhere in the mill and where pulp isretained in contact with peroxide for about three hours. Because ofthese problems, peroxide bleaching is carried out less often thanhydrosulfite bleaching. It must also be mentioned that in practicalterms hydrosulfite bleaching is limited to increasing the brightness ofa mechanical pulp by about 6 to 8 percentage points. If greaterbrightening is desired, a peroxide bleaching procedure must be followed.It is also common in the industry that'a peroxide stage is followed by ahydrosulfite bleaching stage to get higher brightness.

The hydrosulfite bleaching of mechanical pulps has to be regulatedbecause of two characteristics of hydrosulfite. This material is astrong reducing agent and quite stable in an alkaline solution, but inweakly acidic or acidic solutions, it decomposes at a very fast rate.The bleaching of mechanical pulps has to be carried out in an acidicmedium because of the darkening of mechanical pulps at pH above 6.5.Considering the above two factors, laboratory experiments have shownthat the optimum pH range for hydrosulfite bleaching of mechanical pulpsis 5.5 to 6.0. In the mills, therefore, where the pH of mechanical pulpsis in the 4.0 to 4.5 range, the addition of a buffering agent isnecessary to raise the pH although it normally does not reach theoptimum range.

Another problem with hydrosulfite bleaching is the catalytic effect ofheavy metal ions on the rate of decomposition of the bleaching agent. Tocounteract this effect, addition of sequestering agents to themechanical pulp during bleaching is desirable. The most common ma=terial added for this purpose is sodium tripolyphosphate (STPP), asdescribed in US. Patent 2,707,144. The use of some other materials hasalso been patented, e.g., ethylenediaminetetracetic acid (EDTA) and(EDTA) trisodium salt (U.S. Patent 2,707,145), sodium citrate with orwithout alkali tetraborate (U.S. Patent 2,826,478) and several inorganicsalts (U.S. Patents 2,071,304 and 2,071,307). However, because of costconsiderations, the

most common reagent used is STPP.

In a technical bulletin issued bydu Pont (vol. 11, No. 3, pages 13613-8,September 1955) a process is described in which a mechanical pulp isbleached with 1% sodium hydrosulfite on pulp in the presence of 0.5%STPP on pulp or 0.1 to 0.2% EDTA 0n pulp. The pH of the pulp ismaintained at 5.5-6.0 using approximately 0.1% sodium silicate. Theyalso mention that if sodium silicate is not required for pH control,0.01% silicate is recommended for corrosion control.

Although STPP is quite satisfactory for the above use, it is lost in theeffluent waters of the mill. On hydrolysis it yields the phosphate ionwhich, in combination with carbon and nitrogen, is believed to beresponsible for algal growth in natural water systems, and the watersbecame fouled by a green scum. Hence, for ecological considerations itis essential to replace ST PP by a suit= able substitute. The cost ofother satisfactory materials such as EDTA, sodium citrate, etc. is ahindrance to their use.

Therefore, it is an object of the present invention to provide a processfor the bleaching of mechanical pulps.

It is another object of the present invention to provide a hydrosulfitebleaching process wherein sodium tripolyphosphate, which isconventionally used at present, is completely eliminated, and isreplaced with a compound which will not result in the adverse ecologicaleffects described above.

Other objects and advantages of the present invention will be apparentfrom the detailed description of the invention which follows.

It has been found that the ecological problems attendant thehydrosulfite bleaching of mechanical pulps in the presence of sodiumtripolyphosphate can be avoided completely by eliminating the use ofsodium tripolyphosphate and using in its stead an alkali metal silicate,such as, for example, sodium, potassium or ammonium silicate. Althoughthe invention will hereinafter be illustrated by reference to sodiumsilicate, it will be appreciated and understood that the other alkalimetal silicates can also be used. In the description of the inventionthe amounts of sodium silicate are expressed as 41 B. solution ascommonly used in the pulp and paper industry. The results when usingeither solid sodium silicate or any of the other alkali metal silicatesare identical.

Mechanical pulp of standard production, either stone or refinergroundwood, has a fiber concentration within the range of from about 1%to about 15%. Based upon equipment limitations and to expedite handling,it is preferred, however, to employ an aqueous pulp suspension having aconsistency of from about 1% to about 6%, based on the weight of themoisture-free pulp, and even more preferably a pulp having a consistencyof from about 3% to about 4%. The consistency of 3 to 4% mentioned aspreferable is usually the consistency in the deckers where thehydrosulfite bleaching is normally carried out in the mill.

Either zinc or sodium hydrosulfite may be employed in accordance withthe present invention, in a concentration ranging from about 0.05 toabout 1.0%, based on the weight of moisture-free pulp. In most cases itis preferable to use from about 0.3% to about 0.5%, by weight, of sodiumor zinc hydrosulfite.

In the preferred embodiment of the present invention, sodium silicateand the hydrosulfite are mixed with the aqueous suspension of mechanicalpulp and heated for a period of from about 10 minutes to about 6 hoursat from about room temperature to about 100 C. Prefferably, however, thereaction is carried out at a temperature of from about 50 to about 70 C.for a period of from about 1 hour to about 3 hours.

In another embodiment of the invention, the sodium silicate is firstadded to the aqueous suspension of mechanical pulp and heated for ashort period of time, for example, minutes at 60 C. Then thehydrosulfite is added, followed by stirring and heating at, for example,60 C. for about 1 hour.

When 1% sodium hydrosulfite is employed, less than 0.15% sodium silicatesolution, which is equivalent to about 0.05% solid sodium silicate or 1lb./ton of mechanical pulp, produces no effect. At at sodium silicateconcentration of about 0.15% or above, based on the weight ofmoisture-free pulp, the brightness of the pulp is improved beyond whatis observed when sodium hydrosulfite alone is used. The brightnesscontinues to increase with increasing amounts of sodium silicate. If,however, more than 1% on pulp, i.e., 20 lbs/ton is used, the advantageachieved starts to decrease because the pH of the pulp slurry orsuspension rises above the optimum range for mechanical pulp bleaching.This optimum range is from about pH 5.5 to about pH 6.0. While thisoptimum range produces the best results, a range of from about pH 4.0 toabout pH 6.5 can be employed satisfactorily. However, when the pH isbelow 4 the hydrosulfite begins to decompose and when the pH is above6.5 a darkening in pulp color occurs. Thus, when 1% hydrosulfite isemployed the useful range of sodium silicate is from about 0.15% toabout 1.5%, based on the weight of moisture-free pulp, with from about0.5% to about 1% being preferred for optimum results.

In the Du Pont Technical Bulletin mentioned above, 0.1% sodium silicateon pulp is used only if necessary to adjust the pH between 5.5 to 6.0.Since other sequestering agents, STPP or EDT'A, are present in muchlarger quantities, silicate has no effect on the brightness improvement.In addition, the amount added was too small to have any effect. Becausesilicate is not known to be a good sequestrant, the results obtained inthis invention are unexpected and unobvious. Because of this unobviousresult, it is postulated that the sodium silicate sequesters the ions ofiron, copper manganese, etc.

The use of sodium silicate in peroxide bleaching of mechanical pulps,along with sodium hydroxide and magnesium sulfate, is a well knownprocess. In peroxide bleaching, unlike hydrosulfite bleaching, the pH ofthe pulp slurry is very high, and sodium silicate acts as a buffer tomaintain the high pH. In addition, it reacts with magnesium sulfate andforms colloidal magnesium silicate; the latter adsorbs manganese ionswhich are ex" tremely harmful in peroxide bleaching. Since manganese ionhas no effect on hydrosulfite bleaching of mechanical pulps, the use ifsodium silicate in peroxide bleaching is not indicative of any advantagein hydrosulfite bleaching.

In addition to increased brightness in hydrosulfite bleaching, sodiumsilicate reduces pollution because it replaces the phosphates formed bythe STPP. The effluent water in this case contains unchanged sodiumsilicate. If this effluent is treated with a mineral acid such ashydrochloric acid, sulfuric acid, nitric acid, etc., sodium silicate isconverted into the sodium salt of the acid used and silica. Ifhydrochloric acid, which is the preferred acid, is used, the productsare sodium chloride and silica, two innocuous materials. Simplecalculations show that 1 pound of 41 B. sodium silicate will give aconcentration of 1 p.p.m. sodium chloride in the etfiuent water if thevolume of the efiluent is 20,000 gallons/ton, a reasonable average for anewsprint mill. Silica is not objectionable in any water system.

Use of silicate will also help in corrosion control of mill equipmentmade of alloys of iron, copper and small amounts of other metals. Thesilicates are well known corrosion inhibitors and probably do notrequire further discussion. This corrosion inhibition is furtherenhanced by the increased pH effected by silicate. Since the pH of thepulp slurry i 4.0-4.5, the solution should be corrosive to many metals.Any increase in pH should decrease the corrosivity of the solution. Itmay, however, be mentioned that phosphates also have a similar effect,although the corrosion inhibition of phosphates is less marked than thatof silicates.

To assess the usefulness of the invention in a mill operation, a milltrial was run using 15 to 20 pounds of 41 B. sodium silicate per ton ofstone groundwood. An unrelated and unexpected result was noticed. It wasfound that the energy required for grinding the wood with stone grinderswas reduced by 5 horsepower day/air dry ton of groundwood, whichcorresponds to 6-7% of the total energy used. This is a substantialsaving in the energy consumption of the mill. This advantage of powerrequirement reduction is opposed to what is observed with STPP. Use of3-4 pounds STPP per ton of groundwood usually raises the powerrequirements by about 3-4 horsepower days/ air dry ton of mechanicalpulp.

EXAMPLE 1 A 40 gram (25% solids content) sample of commercial stonegroundwood prepared from a mixture of black spruce and balsam fir wassuspended in 303 ml. of water by stirring to give a suspension ofgroundwood in water of 3% consistency. The slurry was heated to 60 C. ina constant temperature water bath during five minutes. The

heated slurry was treated with 0.1 gram sodium hydrosulfite and theclosed jar shaken for 30 seconds to mix the pulp slurry with bleachingagent. An immediate brightening eifect was visible. To complete thebleaching the jar was then replaced in the 60 C. water bath and heldthere at 60 C. for one hour. After this interval, the pulp was filteredoff and washed with water. Two handsheets weighing 5 grams each and 12.5cm. in diameter were prepared from the original unbleached pulp and twosimilar sheets were prepared from the bleached pulp. The sheets werepressed between cellulose blotters and stainless steel plates for twominutes at a pressure of 240 lbs/square inch. After drying between ringsfor 16'hours, their brightness (reflectance) was measured using anElrepho reflectance meter. Relative to the magnesium oxide standard witha reflectance of 100% the unbleached pulp had a brightness of 59.6% andthe bleached pulp 64.2%. Hence treatment of this groundwood with 1%sodium hydrosulfite increased the brightness by 4.6 percentage points.

EXAMPLES 27 A 40 gram (25% solids content) sample of commercialgroundwood (same as that used'in Example 1) was suspended in 303 ml. ofwater containing various amounts of 41 B sodium silicate (0.2 to 1.0% byweight on moisture-free pulp) by stirring to give a suspension of 3%consistency. This suspension was heated during five minutes to 60 0.,treated with 100 mg. sodium hydrosulfite (1% by weight on moisture-freepulp), mixed and heated at 60 C. for one hour. After this interval thepulp was filtered, washed with water and two handsheets made asdescribed in Example 1. The brightness of the unbleached pulp was 59.6%.Table I gives the brightness of the sheets after treatment with variousamounts of 41 B. sodium silicate (or equivalent solid sodium silicate)from which the amount of brightness gain due to sodium silicate can becalculated knowing that 1% sodium hydrosulfite alone TABLE I Bright- 41B. Solid ness of Brightsodium sodium bleached ness gain silicatesilicate pulp (per- (percent- Bright- Example (percent (percent cent ona e ness number on pulp) on pulp) pulp points gain 1 1 Due to sodiumsilicate (percentage points).

EXAMPLE 8 A 40 gram sample of (25% solids content) groundwood (same asused in Example 1) was suspended in 303 ml. of water containing mg. ofsodium tripolyphosphate (0.2% by weight on moisture-free pulp) to give a3% consistency. It was bleached with 100 mg. sodium hydrosulfite as inExamples 2 to 7. The brightness of the bleached pulp was 64.8% showing abrightness gain of 5.2 percentage point due to sodium hydrosulfite andsodium tripolyphosphate. Since the same amount of sodium hydrosulfitealone produces a brightness gain of 4.6 percentage points, thebrightness gain due to 0.2% sodium tripolyphosphate is 0.6 percentagepoints.

EXAMPLE 9 A 40 gram sample of (10 gram moisture-free weight) ofgroundwood was bleached exactly as in Example 8 but with 40 mg. sodiumtripolyphosphate. The bleached pulp had a brightness of 65.5% whichrepresents a total brightness gain of 5.9 percentage points. Since 1%sodium hydrosulfite produces a 4.6 percentage point gain (Example 1),0.4% sodium tripolyphosphate produces an extra 1.3 percentage point gainin brightness.

6 EXAMPLES 1045 In Examples 2 to 7 the amount of sodium hydrosulfite was1% on pulp. In Examples 10 to 15 experiments were carried out with 0.5%sodium hydrosulfite in the presence of 0 to 1.0% 41 B. sodium silicateon pulp. The advantage of adding sodium silicate is evident from theresults shown in Table II.

TABLE II Bright- 41 Be Solid ness 0! Brightbleached ness gain pulp (per-(percent- Brightcent on age ness pulp points) gain 1 See footnote 1bottom of Table I.

EXAMPLE 16 The same sample of groundwood was bleached with 0.5% sodiumhydrosulfite and 0.2% sodium tripolyphosphate. The brightness of thebleached pulp was 63.2 which represents a brightness gain of 3.6 points.Since 0.5% sodium hydrosulfite produces a brightness gain of 3.1percentage points (Example 10) the gain because of 0.2% sodiumtripolyphosphate should be 0.5 percentage points.

. EXAMPLE 17 When the experiment of Example 16 was repeated with 0.5%sodium hydrosulfite and 0.4% sodium tripolyphosphate, the bleached pulphad a brightness of 63.9, i.e., a total brightness gain of 4.3percentage points. Hence the gain in brightness due to 0.4% sodiumtripolyphosphate is 4.3 minus 3.1, or 1.2 percentage points.

EXAMPLE 18 A sample of commercial refiner groundwood prepared byrefining a mixture of chips of black spruce and balsam fir was bleachedwith 1% sodium hydrosulfite as in Example 1. The brightness of pulp rosefrom 58.5 to 63.0 indicating a gain of 4.5 percentage points.

EXAMPLE 19 When the experiment described in Example 18 was repeated with1% sodium hydrosulfite and 0.5% 41 B. sodium silicate, the brightnessrose from 58.5 to 63.5, a total gain of 5.0 percentage points. Since 1%sodium hydrosulfite producesa gain of 4.5 points (Example 18) thegaindue to 0.5% 41 B. sodium silicate is 0.5 percentage points.

EXAMPLE 20 In a mill trial beginning with the 3rd day and continuingthrough the 6th day, 15 to 20 pounds (0.75% to 1.0%) of 41 B. sodiumsilicate per ton of groundwood pulp was added at the deckers. Sodiumsilicate addition was discontinued on the 6th day. During the trial 4 to8 pounds of sodium hydrosulfite (0.2% to 0.4%) per ton of pulp wasemployed. The power consumptions were recorded every day.

Table III shows the advantage of adding sodium silicate in reduction-ofpower consumption. The data also indicate that'there is some time lagbetween the addition of silicate and appearance of the effect. Also,since silicate remains in the closed water system after its addition hasbeen stopped, it advantageous effect continues for some time.

TABLE III Power requirements Day: v HPD/ADT 1 1st 82.5 2nd 80.1

1 Horse power days/air dry ton of pulp. 9 Silicate started. Silicatestopped.

What is claimed is:

1. A process for the bleaching of mechanical pulps consistingessentially of reacting an aqueous suspension of said pulp with acompound selected from the group consisting of sodium and zinchydrosulfite in the presence of from about 0.05 to about 0.562% of asolid alkali metal silicate, based on the moisture-free weight of thepulp.

2. The process of claim 1 wherein the reaction is carried out at atemperature of from about room temperature to about 100 C. for a periodof from about 10 minutes to about 6 hours.

3. The process of claim 2 wherein the reaction is carried out at atemperature of from about 50 C. to about 70 C. for a period of fromabout 1 hour to about 3 hours.

4. The process of claim 1 wherein the consistency of the pulp in theaqueous suspension is from about 1% to about 6%, based on themoisture-free weight of the pulp.

5. The process of claim 4 wherein the consistency of the pulp in theaqueous suspension is from about 3% to about 4%, based on themoisture-free weight of the pulp.

6. The process of claim 1 wherein the hydrosulfite concentration is fromabout 0.05% to about 1.0%, based on the moisture-free weight of thepulp.

7. The process of claim 1 wherein the alkali metal silicate is sodiumsilicate.

8. The process of claim 7 wherein the sodium silicate is 41 B. sodiumsilicate and its concentration is from about 0.15% to about 1.5%, basedon the moisture-free weight of the pulp.

9. The process of claim 8 wherein the concentration of the 41 B. sodiumsilicate is from about 0.5% to about 1.0%, based on the moisture-freeweight of the pulp.

10. The process of claim 1 wherein the pH of the aqueous pulp suspensionis from about 4.0 to about 6.5.

11. The process of claim 10 wherein the pH of the aqueous pulpsuspension is from about 5.5 to about 6.0.

References Cited UNITED STATES PATENTS 2,862,784 12/1958 Kise et al.16280 X 2,187,016 1/1940 Craig 16271 X 2,071,304 2/1937 Hirschkind 16271OTHER REFERENCES Du Pont Technical Bulletin, September 1955, p. 136.

S. LEON BASI-IORE, Primary Examiner A. L. CORBIN, Assistant Examiner US.Cl. X.R. 8--Il0; 162-80, 83

